1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser of selective oxidization type, and more particularly, to a structure for controlling the optical power of a multimode surface emitting semiconductor laser.
2. Description of the Related Art
Recently, there has been an increased demand for a surface emitting semiconductor laser that can easily realize an arrangement of a two-dimensional array of sources in the technical fields of optical communications and optical recording. Such a surface emitting semiconductor laser is also called a vertical-cavity surface-emitting laser (VCSEL). The VCSEL has many advantages of a low threshold voltage, low power consumption, easy making of a circular spot of light and good productivity resulting from wafer-based evaluation.
Nowadays, low-cost multimode optical fibers, which are typically plastic optical fibers (POF), are developed, and short-distance optical communications as short as a few meters to several hundreds of meters are getting the attention. Generally, long-distance optical communications employ the combination of a single-mode type optical fiber and an edge emitting semiconductor laser having a relatively long wavelength of 1.31 μm or 1.55 μm. However, these components are expensive and are unsuitable for applications in local areas that should be less expensive.
An optical source used for the multimode optical fiber is required to be less expensive and avoid the use of a special optical system or driving system. The surface emitting semiconductor laser may satisfy the above-mentioned requirements and is one of the powerful options as the optical source for the multimode optical fiber. In order to use the surface emitting semiconductor laser as the optical source in optical communications utilizing the multimode optical fiber, it is desired to stabilize the oscillation mode and reduce jitter components contained in the optical output.
Japanese Laid-Open Patent Application Publication No. 2001-156395 discloses a selectively oxidization type surface emitting semiconductor laser in which an optically transparent layer is formed in a beam emitting aperture for controlling the oscillation transverse mode. The transparent layer may, for example, be a dielectric film, and functions to reduce the reflectance of an area that does not desire laser oscillation, so that the oscillation transverse mode can be controlled.
Japanese Laid-Open Patent Application Publication No. 9-326530 proposes a VCSEL intended to reduce the jitter (fluctuation in the turn-on delay time) of the laser device. A diffusion reinforcement region provided in the vicinity of the active layer is doped with an acceptor impurity of a high concentration, so that the number of holes induced in the quantum well region is approximately one-digit larger than the number of electrons therein. This increases the diffusion rate in the quantum well region.
However, the conventional selectively oxidization type semiconductor lasers have the following problems to be solved. FIGS. 7A and 7B respectively show a beam profile and an optical output profile in a far-field image obtained when the multimode surface emitting semiconductor laser is used as an optical source. A symbol L denotes the distance from the optical source to the far-field image, and θ denotes the divergence angle of the diameter of the beam in the far-field image viewed from the center of the optical source. Generally, this kind of surface emitting semiconductor laser has a profile like a doughnut shape. More particularly, the laser beam of the fundamental mode exists within a spread or divergence angle θ0 from the center of the optical source, and the beam profile is indicated by P0. When the divergence angle is greater than θ0, the resultant laser beam is of a high-order mode in which the first-order mode or a higher-order mode in addition to the first-order mode is combined with the fundamental mode. The beam profile of the high-order-mode laser beam is indicated by P1. The optical output in the high-order mode indicated by P1 is grater than that of the fundamental mode. In other words, the outer portion P1 of the doughnut-like profile involved in the first-order or higher-mode is brighter than the inner portion P0 involved in the fundamental mode. The laser emissions in the oscillation modes do not have the same profiles and do not occur concurrently. Thus, there is a difference in response between the fundamental-mode light and the first-order or higher-order light. The difference in response causes the jitter in the optical signal when the multimode surface-emitting laser is used as the optical source.
FIG. 8A shows an eye pattern that serves as an index for checking the quality of the laser beam. The eye pattern is a pattern in which, when the laser beam is modulated (turned on and off), the modulated light signal is superimposed at random. The horizontal axis of the eye pattern denotes the time, and the vertical axis denotes the optical power. The eye pattern shown in FIG. 8A is an example observed when a multimode VCSEL capable of outputting light that has an optical power of 3 mW and a wavelength of 850 nm is used as the optical source. It can be seen from the eye pattern of FIG. 8A that each emission in the respective oscillation mode does not occur concurrently, so that a jitter J takes place. The jitter J is the time difference in response between the optical signals in the oscillation modes. The eye pattern ideally converges on a single line. If the optical output contains a jitter that exceeds a threshold time, the rate of incidence of error contained in the optical signal on the receive device side will considerably increase. This restrains the maximum bit rate achievable in the data link.
The aforementioned transparent layer arranged in the vicinity of the beam emission aperture proposed in Japanese Laid-Open Patent Application Publication No. 2001-156395 is not intended to reduce the jitter in the multimode laser beam but to control the oscillation transverse mode. The VCSEL proposed in Japanese Laid-Open Patent Application Publication No. 9-326530 does not have any mechanism for reducing the jitter.